This invention relates to the mounting and connection of integrated circuit elements.
It is common practice for the manufacturers of integrated circuit elements or chips to provide to the user an encapsulated unit of a standard size with a standard pin spacing. The unit may thus be connected simply into a suitable printed circuit. Such an encapsulated unit may contain one, or several interconnected, chips and may provide a single function such as a shift register or adder, or multiple functions such as several independent amplifiers.
The operating speed of the circuits which may be made on chips is so high that the time delay due to the propogation of electrical signals along even 2 or 3 cms. of wire is comparable with the time delay of a logic gate or the switching time of a flip-flop. Since the encapsulated circuit involved wire lengths of this order, thought has been given to other methods of construction which permit much higher densities of chips and interconnecting wiring, thus reducing the average length of the signal wiring. A further problem which is likely to arise with an increase in chip density is the need to provide for the rapid dissipation of the heat which is generated by the chips when in operation.
A solution to these problems is described in U.S. Pat. No. 3390308. A number of chips are accommodated in depressions in a substrate which may be of anodised aluminium or other good heat conducting material. A dielectric sheet is formed with windows corresponding to the positions of the chips and with conductive beam leads which project into the window area. The sheet is placed over the substrate and the beam leads are welded to the contact pads on the chips. It will be appreciated that some number of interconnections may be provided on the dielectric sheet, but that, if this number is insufficient, further sheets will have to be provided on top of the first sheet to form a multi-layer printed circuit of conventional form. The result of this is that the chips are now buried beneath the printed circuit and replacement of a faulty chip is virtually impossible.
Furthermore, whilst the task of welding a beam lead to a contact pad is not unduly difficult with present techniques, but the welding of many such connections whilst maintaining mechanical and electrical integrity is time consuming and costly.